This invention relates to a glass-ceramic suitable for use as a high strength material requiring high resistivity to heat and impact and as materials requiring a fine processing or a precision processing such, for example, as electronics parts, magnetic disk substrates and machine parts. The invention relates also to a method for manufacturing such glass-ceramic.
Known in the art are various types of cordierite glass-ceramics obtained by melting and subjecting to a heating treatment a base glass of a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO system including a nucleating agent. These glass-ceramics generally contain a cordierite crystal phase (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18) as the predominant crystal phase.
There has recently been an increasing tendency toward requirement for the following properties for the above described materials:
(1) The glass-ceramic should have fine crystal grains and have excellent precision processing and excellent fine polishing properties and also have a high mechanical strength. PA1 (2) The glass-ceramic should not substantially contain Na.sub.2 O or K.sub.2 O ingredient because ion diffusion of these ingredients during the processing of th material tends to change the required properties of the glass-ceramic. PA1 (3) Melting and refining of the base glass should be easy enough to prevent occurrence of cords, foams and inclusions in the glass-ceramic and thereby enable the glass-ceramic to maintain optical homogeneity of a high degree. PA1 (4) The crystallizing temperature of the base glass should be relatively low and thereby a stable mass production of the glass-ceramic can be achieved.
As prior art of cordierite glass-ceramics, for example, Japanese Patent Application Laid-open No. Sho 55-37496 discloses a glass-ceramic obtained by adding La.sub.2 O.sub.3, Pr.sub.2 O.sub.3 and Nd.sub.2 O.sub.3 in a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO glass including TiO.sub.2 as a nucleating agent. U.S. Pat. No. 4,304,603 and Japanese Patent Application Laid-open No. Sho 33-7543 disclose glass-ceramics of a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO glass including TiO.sub.2 as a nucleating agent. Japanese Patent Application Laid-open No. Sho 63-112439 discloses a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO glass including TiO.sub.2 and ZrO.sub.2 as nucleating agents.
These prior art cordierite glass-ceramics all require high temeratures ranging from about 1520.degree. C. to 1650.degree. C. for melting base glass materials and, for this reason, it is difficult to obtain a base glass having an excellent homogeneity. These prior art glasses require also high temperatures of about 1100.degree. C. to 1345.degree. C. for crystallization and hence stable mass production of these glass-ceramics is extremely difficult and also it is inevitable in these prior art glass-ceramics that undesirable coarse crystals grow due to crystallization at a high temperature.
It is, therefore, an object of the invention to provide a cordierite glass-ceramic which has excellent melting and crystallizing temperature properties and thereby eliminates the above described disadvantages of the prior art cordierite glass-ceramics while maintaining the above described requirements for the cordierite glass-ceramic.